Typical image forming processes in electrophotography or electrostatic printing comprise a developing step for uniformly charging a photoconductive insulated layer, exposing the insulated layer, dispersing the charges on the exposed portions to form an electrical latent image, and adhering charged fine toner particles on the latent image to make it visible; a transferring step for transferring the visible image thus obtained onto a transfer material such as transfer paper; and a fixing step for permanently fixing it by heat or pressure.
Various performances are required in each of the steps mentioned above for toners and binder resins for toners used in electrophotography or electrostatic printing. For instance, toners and binder resins for toners have to maintain an amount of electrostatic charge appropriate to copying machines without being affected by the surrounding environment such as the temperature and humidity to adhere toners on electrical latent images at the developing step. Also, the anti-offset property, that is, the property of not-adhering to heated rollers, and the fixing property on papers must be excellent at the fixing step in a heated roller fixing method. Further, a blocking resistance, that is, the property of toners not blocking during storage, and excellent image characteristics are also required.
Heretofore, styrene-acrylic type resins have been widely used. Specifically, linear type resins and cross-linked type resins have been used as binder resins for toners. In the linear type resins, resins are known which are prepared by blending a polymer of a high molecular weight with a polymer of a low molecular weight to improve the fixing property and anti-offset property. In the cross-linked type resins, improvements in the fixing property and anti-offset property are being made by broadening the molecular weight distribution by cross-linking. Particularly, considerable research is being carried out on linear type resin. As described in the specifications of Japanese Examined Patent Publication No. 63-32182, Japanese Unexamined Patent Publication No. 62-9356, and others, attempts are being made to improve the fixing property and anti-offset property by controlling the region and molecular weight high molecular weight and low molecular weight resins, respectively. Also, as to the image characteristics, vivid images are being obtained without fogging by controlling the pulverizability of resins to prevent toners and binder resins for toners from overpulverization at the time of printing by means of the blending ratio of the high molecular weight polymer to the low molecular weight polymer.
However, the balance between the fixing property and anti-offset property cannot necessarily be obtained only through the blending of polymers with different molecular weights or through the control of the high molecular weight region and low molecular weight region as well as the molecular weight of resins. Further, copying machines are being increased in speed year by year. Improvement in the fixing property is sought to cope with this through further decrease of the molecular weight of the lower molecular weight polymers. However, toners using toner resins lowered in molecular weight have a low mechanical strength, toners charged through friction with carriers is high speed printing are overpulverized, fogging is produced in the images after printing, and other problems occur in the image characteristics.
With respect to this point, attempts have been made, as described in the specification of Japanese Examined Patent Publication No. 3-48506, in which a relatively high molecular weight polymer is blended to suppress the overpulverization of toners, a super-high molecular weight polymer is blended to improve the anti-offset property, and a low molecular weight polymer is blended to improve the fixing property. However, a satisfactory fixing property cannot be obtained since a relatively high molecular weight polymer and a super-high molecular weight polymer are blended.
Also, with regard to the image characteristics, attempts have been made, paying attention to the charging step which precedes image formation, to introduce an acid monomer to improve the charge buildup as a method for obtaining charges more stably. The introduction of an acid monomer, however, increased the acid value of the resins, and stabilized charge characteristics were not obtained due to the effect of environmental stability, particularly, that of humidity. Further, attempts have also been made to decrease residual monomers and a residual solvent contained in the resins to prevent images from fogging. However, it was difficult to obtain sufficiently vivid images only through the decrease of the residual monomers and solvent.